Yu Menglong says that during the Tianbao era of the Tang Dynasty — the reign of Emperor Xuanzong, around 751 AD — he was bestowed the title of Dragon King of the East Sea by imperial decree. The East Sea under his jurisdiction lies off the eastern coast of China, north of Taiwan and west of Japan's Ryukyu Islands, covering a total area of approximately 770,000 square kilometers — an immense domain.

As Dragon King of the East Sea, Yu felt the territory was too vast to govern alone, and petitioned the Emperor for an additional administrator to assist him. One can appreciate just how demanding his responsibilities were: he oversaw all life in the mountain waters — lakes, ponds, rivers, and streams — as well as all life in the sea itself, including fish and merfolk. He managed the work of rainfall, purifying seawater into fresh water to irrigate the land, and handled various other duties assigned by the Jade Emperor.